Classfication Of The SST Interannual Variability Major Modes Of Tropical Indian Ocean And Their Relations With ENSO

Based on observational datadets, multiple coupled ocean-atmosphere general circulation models and various statistical methods, multiple types of Tropical Indian Ocean (TIO) interannual variability and their relations with Tropical Pacific (TP) are investigated. By comparing the observations and coupled models, the simulation of the correlations between El Nino-South Isolation (ENSO) and Indian Ocean Basin-wide (IOB)/Indian Ocean Dipole (IOD) are studied; after confirming the simulation of the relations between ENSO and IOB/IOD, the IOB and IOD events are classified into diferent types to investigate in the CMIP5 models taking advantage of enough samples of the control run simulation. It is revealed that there are similarities and differences in the physical processes for different types of modes; so then an evolutionary chain is pointed out for the first time, which connects the interannual interactions and translation processes in the TP and TIO together; some new results are found as follows:1. Based on the persistance of IOB mode, it is the first time to classify the IOB mode into three types:the first type of IOB mode sustains till Septempber and than dispears, which is named sustained IOB; the second type of positive/negative IOB (pIOB/nIOB) mode transforms into positive/negative IOD (pIOD/nIOD) mode, which is called inphase transformable IOB; the third type of pIOB/nIOB mode transforms into nIOD/pIOD mode, which is called outphase transformable IOB. Their propobalities are 38%(53%),29%(23%) and 33%(24%) in observation (model) respectively. It is revealed that the Australia high pressure in May/June largely influences the surface wind anomalies and local SSTAs to the north of Austrilia, and leads to variations in the SST corss-equatorial gradient and air-sea interaction there, which cause the IOB events evolve differently in June:sustain, transform into inphase pIOD or outphase nIOD.2. To fix the argue about whether IOD mode is dependent or independent of ENSO, according to different trigger conditions, the IOD events with peak in fall are categorized into three types:the first type is closely related to the development phase of El Nino (La Nina), and they are about 40% of the total IOD events; the second type is evolved from the basin-wide warming (cooling) in the TIO, usually occurring in the year following El Nino (La Nina) (30%); the third type is independent of El Nino (La Nina)(10%). The dominant trigger condition for the first (third) type of IOD is the anomalous Walker Circulation (anomalous cross-equatorial flow); the anomalous horizontal SST gradient in the TIO (north of Australia and South East China Sea) in spring is the trigger condition for the second type. Though the trigger conditions are different for the three types of IOD, their developing processes are all affected by the internal air-sea interaction within TIO.3. The "Gear" between Indian and Pacific Ocean ("GIP") coupling effect (Wu and Meng,1998) does exist but with conditionality. The "GIP" coupling effect applies to the first type of IOD, which coocurrence with ENSO. But not for the second type of IOD, whose Walker Circulation anomaly in the Indian Ocean strengthens with weakening of Walker circulation anomly in the Pacific. The "GIP" coupling effect is also not applicable for the third type of IOD, which occures witout Walker Circulation anomaly in the Pacific.4. Taking the advantage of more samples in ctrol run simulation for 500 years. It is first pointed out that not all IOB and IOD events have close relation with ENSO: for some El Nino (La Nina) events with weak intensity and late occurrence (after June), the pIOD(nIOD) mode will not appear during the El Nino (La Nina) developing phase, and there is also no pIOB(nIOB) mode during the second year of El Nino (La Nina); in contrast, for the El Nino (La Nina) with larger amplitude and early occurrence, there is an evolutionary chain, connecting the TP and TIO interannual variaibility major modes together, and reflecting their interactions and translations: there are about 40% El Nino appearing before May, which will trigger the occurrence of the first type of pIOD in June. This type of pIOD and El Nino developes in mutual promotion and cceleration, and cause the appearance of pIOB mode. When the pIOB mode peaks after El Nino’s peak, it may sustain, transform into inphase pIOD or outphase nIOD in June of the second year of El Nino. The inphase transformable pIOB events constraint the translation from El Nino to La Nina, but the sustained and outphase transformable pIOB events are both in favour of the translation from El Nino to La Nina. The evolutionary chain reflects that the pIOB or pIOD mode always co-evolves with El Nino and interacts with each other during the evolutionary process of El Nino from developing to decaying or transferring into La Nina. Vice visa.